Devices for the delivery of medicament in an automatic way e.g. auto-injectors, are known as convenient and safe aids for patients to administrate various drugs themselves. For safety reasons, many devices for the delivery of medicament include covers and other devices that protect users before and after use, for example, from an injection needle Although different devices for the delivery of medicament vary in their total feature sets, they all have a mechanism that delivers the contents of a preloaded, prefilled container automatically, i.e., without requiring a person to manually force the contents within a container through a delivery member e.g. a needle, a nozzle, into the patient.
Autoinjectors are described in U.S. Pat. No. 5,478,316 to Bitdinger et al.; U.S. Pat. No. 7,112,187 to Karlsson; and U.S. Pat. No. 7,125,395 to Hommann et al.; U.S. Patent Application Publication No. 2007/0021720 to Guillermo; and International Publication No. WO 2006/057604 A1 by Olson, for example. The automatic delivery mechanism in a device for the delivery of medicament usually includes a compressed helical spring that drives a plunger rod forward as the spring decompresses upon activation of the device. In many cases, such springs work well. A fully compressed helical spring provides a force that is large enough to overcome the static friction between the plunger and the inner wall of a container, so called break loose force, and the spring at its full extension, which is often not at its full decompression, provides a force that is large enough to complete the injection stroke but causing the probability of the container breakage.
A challenge in the design of such springs is balancing the need for sufficient force at the end of stroke against the need for a not-too-large force during storage (when the spring is fully compressed) that may overload the other components in the device. Such other components may be made of plastics or similar materials that have limited strengths.
Thus, the drive mechanism in such a device for the delivery of medicament should meet the following simultaneous goals: 1) exert enough force to overcome the stopper's “break loose” force and initiate delivery of the medication, 2) exert enough force to complete-the injection stroke, 3) meet goals 1) and 2) in an acceptable time frame (usually a few seconds), and 4) exert a low force during storage.
Rather than use typical helical springs, some delivery devices use constant force springs. U.S. Pat. No. 5,478,316 cited above describes such a device.
Although constant-force springs are better able than compression helical springs to meet the design goals of such a device, they also come up short. During the processing of a prefilled container, the container is typically siliconized, i.e., given a coating of silicone, to ease the movement of the stopper during delivery. The silicone can be applied in a few different ways but all strive to distribute the silicone evenly—especially at the end of the container nearest the delivery member. Usually the silicone coating is thinner or lacking at the end of the barrel, with the result that the stopper's glide force increases towards.
Moreover, EP 0953122 A1 describes a coil spring of strip material having two portions of a non-uniform width and wherein each portion has a constant force characteristic. In the application of said coil spring to an injection device, the spring has a wide portion wound innermost which urges a syringe forwards for auto-penetration i.e. the penetration of the needle into an injection site, and a narrow portion which then takes over to eject the dose i.e. the auto-injection. The impelling of the syringe forwards has a greater force than that applied subsequently to a stopper of the syringe.
Although a coil spring of strip material having portions of a non-uniform width are better able than compression helical springs to meet the design goals of an autoinjector, they also come up with drawbacks. The design and the application of such a spring as described in EP 0953122 A1 leads to the drawbacks of painful penetration since a great force is used for this step, and of not exerting enough force to overcome the break-loose force at the end of the injection stroke.